1. Field of the Invention
This invention relates generally to fabrication of semiconductor devices and more particularly to a two step process for forming a self-aligned contact, preferably composed of polysilicon, which eliminates the key hole problem in the IPO (inter-poly oxide) layer.
2. Description of the Prior Art
The use of self aligned contact (SAC) processes has resulted in higher performing, lower cost, and increased density semiconductor devices. In a self-aligned contact process, spacers are formed on the sidewalls of conductive structures (such as gates and bit lines). An IPO layer is formed over these conductive structures. A contact opening is etched through the IPO. A conductive layer, such as for capacitor crowns, is formed over the conductive structures and in the contact opening. However, as device dimensions and die sizes continue to decrease for higher density, the space between adjacent conductive structures becomes narrower. The surface of the sidewall spacers facing the contact opening become concave which leads to voids or keyholes in the subsequently formed IPO layer. The key holes can fill with conductive material during subsequent formation of a conductive contact layer causing two separate devices (such as capacitors) which are formed over the IPO layer to short. The short can result in cell failure.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering the following patents.
U.S. Pat. No. 5,480,814 (Wuu et al.) shows a SAC with a barrier layer formed of polysilicon and a contact layer formed of a silicide.
U.S. Pat. No. 5,763,303 (Liaw et al.) shows a SAC with a polysilicon layer, a silicide layer and a contact.
U.S. Pat. No. 5,795,827 (Liaw et al.) shows a SAC with triple polysilicon.
U.S. Pat. No. 5,843,815 (Liaw) shows a SAC with a halo implant.